The present invention relates to a method and apparatus for discharging gas out of metal molds, and more particularly to such method and apparatus in a die-casting.
A gas discharging technique has been conventionally proposed for removing gas from metal molds in order to produce a voidless casted product by an injection molding apparatus. To this effect, a gas vent valve is provided for selectively communicating a mold cavity with atmosphere. For example, Japanese Utility Model Application Kokai No. Sho. 61-195853 discloses a technique in which a vacuum pump is connected to the gas vent valve for decompressing the mold cavity during injection molding to suck gas from the cavity. In die-casting, cooling water is sprayed onto the metal molds after opening of the metal molds for cooling the same, and release agent is coated on surfaces of the metal molds.
However, if the metal molds still have high temperature after closure of the metal molds, the release agent in the metal molds or residual cooling water may be converted into contamination gas. If molten metal is poured into a shot sleeve in accordance with an injection start signal, the generation of the contamination gas is greatly promoted due to high temperature of the molten metal. Since the contamination gas contains water content, vapor explosion may occur during filling of the molten metal into the metal molds to degrade fluidity thereof, which causes gas involvement and insufficient run, to thus produce low grade product.
In order to overcome these drawbacks, according to a Japanese Patent Application Kokai No. sho.60-127063, hot air is supplied into the metal molds through the gas vent valve for escaping the gas in the metal molds through a pouring port of the shot sleeve.
However,in the Japanese Patent Application Kokai No. sho.60-127063, contamination gas cannot be sufficiently discharged out of the pouring port, since the gas in the metal molds are scattered in all directions when pressurized hot air is blown thereinto. Further, there are insufficient gas escaping routes in a metal mold portion where a blind alley or dead-end street are provided like a by-pass passage, which in turn makes gas discharge difficult. In an actual casting, the hot air blowing must be terminated before the molten metal is poured into the shot sleeve. However, residual water or oil non-vapored may be vapored and filled in the metal molds after termination of the hot air blowing.
A Japanese Patent Application Kokai No. sho.58-84658 discloses a vacuum die-casting method. The method includes scavenging step where air is introduced into the metal molds through a pouring port of an injection cylinder because of the application of negative pressure by way of a vacuum tank while the gas vent valve is open and concurrently with the closure of the metal molds. The method also includes the steps of filling the molten metal into the metal molds, and a vacuum casting step where the gas vent valve is again open for a predetermined period after the pouring port is closed by a plunger tip to perform casting.
Further, a Japanese Patent Application Kokai No. sho.60-3959 discloses sealingly maintaining a mold cavity in a decompressed state and forcibly discharging gas from the mold cavity until termination of casting in order to remove gas from the mold cavity, the gas being generated at the time of casting on the premise of employment of a cavity produced by casting sands. To be more specific, a discharge hole connected to a discharging blower is formed at an upper portion of the cavity, and an open end portion of a pouring port is covered with a meltable thin plate. Further, a riser port is formed at the upper portion of the cavity, and the opening of the riser port is covered with the meltable plate. Thus, the cavity is sealed by the thin plate. With this state, decompression to the cavity is performed by operating the discharging blower. During decompression, when high temperature molten metal is poured through the pouring port, the thin plate is melted so that the molten metal can flow into the cavity. When the molten metal is brought into contact with the thin plate on the riser port, the plate is melted to complete casting.
However, in the Japanese Patent Application Kokai No. sho.60-127063, contamination gas cannot be sufficiently discharged out of the pouring port, since the gas in the metal molds are scattered in all directions when pressurized hot air is blown thereinto. Further, there are insufficient gas escaping routes in a metal mold portion where a blind alley or dead-end street are provided like a by-pass passage, which in turn makes gas discharge difficult. In an actual casting, the hot air blowing must be terminated before the molten metal is poured into the shot sleeve. However, nonvapored residual water or oil may be vapored and filled in the metal molds after termination of the hot air blowing.
Further, in the Japanese Patent Application Kokai No.58-84658, gas discharge from the metal molds is performed through suction by a vacuum tank during a scavenging step, and therefore, pressure in the vacuum pump will be increased. Then, the pressure in the vacuum tank must be reduced for performing vacuum suction in the subsequent vacuum casting step. However, if the gas discharge is fully performed in the scavenging step, a large pressure increase occurs in the vacuum tank, and therefore, a relatively long period is required for reducing the pressure in the vacuum tank until start of the vacuum casting in the vacuum casting step. Consequently, the shot cycle is disadvantageously prolonged. Furthermore, after the molten metal is poured through the pouring port into the casting sleeve and the plunger tip closes the pouring port, scavenging cannot be performed. As a result, contamination gas generated by the heat from the molten metal is filled in the metal molds, and the gas cannot be scavenged.
Furthermore, it would be difficult to apply the invention described in the Japanese Patent Application Kokai No.Sho 60-3959 into the die-casting technique. That is, in the die-casting, there are provided the pouring port and gaps between the metal molds, and external cooling water and release agent will be converted into gas, which are filled in the metal molds after closure thereof. In this connection, it would be difficult to maintain decompressed pressure in the mold cavity. Moreover, in the invention described in the above publication, attention is not drawn to the contamination gas existing in the mold cavity before casting, but the decompression and forcible gas discharge are performed so as to discharge gas generated during the casting. It would be structurally impossible to substitute clean air for the contamination gas which has been filled in the mold cavity prior to the casting.